Tech Briefs Magazine - May 2021 - PIT-21

Non-Aspheric Surfaces
Backside
Typically, the non-aspheric backside of
an aspheric lens has a limited influence
on the manufacturability analysis and
cost. For large aspheres this is no longer
true. Obviously, the equipment used
needs to be able to accommodate the
size of the optics. More problematic is
the metrology solution, typically a large
aperture interferometer. If an optical
shop also produces components such as
prisms, beamsplitters, and windows, it
can most likely leverage the existing
equipment. Even so, not many asphere
manufacturers have a standard solution
to measure planar surfaces beyond 10
inches (254 mm).
For convex spherical backsides, the
metrology solutions are even more limited, as investing in the large aperture
interferometer and associated large
aperture transmission spheres is often
cost-prohibitive or unavailable. For both
convex and concave spherical backsides,
a larger diameter goes hand-in-hand
with a larger radius of curvature (RoC).
Typically, the RoC is controlled by moving a stage with the optic mounted along
a rail between the cat's eye position
(where the interferometer's beam contacts a single point on the spherical surface) and confocal position (where the
point focus of the interferometer beam
is at the radius of curvature). Thus, the
range of RoC that can be measured is
limited by the length of the rail.
In addition, the use of test plates for
in-process control is risky and cumbersome for large diameter optics. Not to
mention the same difficulties as mentioned above apply to the manufacture
of the test plates themselves.

Of course, to measure the backside of
an aspheric lens, one could make use of
the available asphere metrology. However,
this makes the manufacturing process
costly and inefficient, as the spherical surface would be competing with the aspheric side for measurement time on an
expensive platform and asphere metrology tends to be more time consuming
and/or require additional skills not typically found in spherical optics craftsmen.
As such, it is typically impractical to take a
quick peek at the spherical surface using
asphere metrology during manufacturing
to monitor the process and adjust process
parameters if necessary.
Diameter
As mentioned earlier, as one of the
last processing steps, the diameter of the
part is edged down to the final diameter.
If the optical shop does not have one or
more dedicated edging machines, or if
they are not large enough to handle the
large diameter, the parts will have to be
edged on the asphere grinding machine. This is both inefficient and
expensive.

Surface Quality and Inspection
Arguably, the number of surface
imperfections created correlates with
the area processed. As such, it is more
difficult to maintain a tight surface
quality tolerance specification on a
larger diameter optic, whether specified using the ISO or MIL standard. In
addition, a larger diameter optic is
more difficult to handle and, as such, at
a higher risk of surface defects due to
mishandling. In addition, surface
inspection is especially cumbersome for
large diameter optics, as they require a
lot of handling.

Blank
The blank can come either as a cutdisk (a disc cut from a rod of adequate
diameter) or a pressing (annealed in custom-made molds). For regular-sized
aspheres, it can be a factor of 3- or 4times more economical to use pressings
for high-volume production, depending
on the exact material. For a large
asphere blank, the material cost becomes
the driving factor over the labor cost, as
the volume increases. As such, pressings
become less advantageous to use for
large asphere blanks, especially considering that pressings have a longer lead time
and are limited to a center thickness of
approximately 40 mm.
Coating
As mentioned previously, with increased size of the optic, it is also likely
that the sagittal height is increased. This
will negatively influence the coating uniformity, so keep in mind that specifying
the same coating uniformity as typical for
regular-sized aspheres on a large asphere
will most likely incur a premium.
Keeping these manufacturing and
metrology considerations in mind allows
optical designs to incorporate large
diameter aspheres into their optical systems. The resulting systems pave the way
for high-power laser applications and
high-throughput light collection systems. Sometimes bigger truly is better.
This article was written by Wilhelmus
Messelink, Director of Technology, Edmund
Optics Singapore; and Shawn Scarfo,
Product Line Manager, Lenses, Edmund
Optics (Barrington, NJ). For more information,
contact
Mr.
Messelink
at
pmesselink@edmundoptics.com, Mr. Scarfo at
sscarfo@edmundoptics.com, or visit http://
info.hotims.com/79413-223.

Micro Molding and Micro-Optics
s is often the case when attempting to
define anything in the area of precision, micro, and nano manufacturing,
trying to set some sort of size parameters
around what are considered " micro " optics is difficult. In general terms, however, micro-optics are typically tiny lenses,
beam-splitters, prisms, light-pipes, and
other optical components in the range of
20 microns to 1 mm in size, or larger optical components with micron features.
Wherever light is involved, microoptics offer a possibility for OEMs to fur-

ther miniaturize a product, increase its
functionality, and/or simultaneously
reduce manufacturing costs. Microoptics are used today in an array of medical applications such as endoscopes and
ophthalmic systems, in industrial inspection equipment, in laser-based devices
and fiber communication networks, in
modern smart phones, and any devices
using digital camera technology, ambient light or proximity sensors.
Typically, OEMs engage specialist
micro molders to assist in the manufac-

Photonics & Imaging Technology, May 2021

ture of customized products. Key in the
selection process is a company with
experience in the field, and an understanding of - and ability to manufacture to - the exacting tolerances
demanded. In addition to quality, however, another key driver is adherence to
often short time-to-market requirements.
Micro-optic components are characterized not just by how small they are,
but also by their flatness, with the surface relief feature depth of diffractive
21

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ToC

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Tech Briefs Magazine - May 2021

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